Electron capture detector having separate ionization and sensing regions



P 1968 J. H. BOCHINSKI ETAL 3,378,725

ELECTRON CAPT URE DETECTOR HAVING SEPARATE IONIZATION AND SENSINGREGIONS Filed April 15, 1964 A n 1 ll] WA \YN II/l),

5:, I fiifi jgfikngm s2 II /34 40 32 22 I ZO 3O RECORDER -l INVENTORSJULIUS aeocumsm JAMES c. STERNBERG T: MUM

ATTORNEY United States Patent 3,378,725 ELECTRON CAPTURE DETECTOR HAVINGSEP- ARATE IONIZATION AND SENSING REGICNS Julius H. Bochinski, Fasadena,and James C. Sternberg, Fullerton, Calif., assignors to BeckmanInstruments, Inc., a corporation of California Filed Apr. 15, 1964, Ser.No. 359,827 7 Claims. (Cl. 315-111) ABSTRACT OF THE DISCLOSURE Anelectron capture detector for use such as in gas chromatography havingseparate ionization and sensing portions with discharge electrodes and agas chemically inert with respect to the detector in the dischargeportion. An anode and cathode are positioned in the sensing portionhaving a gradient established thereacross not exceeding approximately .7of a volt per centimeter per millimeter of mercury pressure therebetweento provide for electron capture operation. A sample gas contained in acarrier, inactive with respect to the sample and having no electroncapture capability, flows past the anode into the sensing portion and isexhausted from a point between the anode and cathode together with theinert gas from the discharge portion.

This invention relates to a detector for use in gas chromatography andmore particularly to an improved form of such a detector adapted tooperate in the electron capture mode.

Many different forms of detectors have been used in the prior art in gaschromatography such as thermal conductivity, hydrogen flame, solutionconductivity, breakdown voltage and photoelectric effect detectors,detectors employing radioactive materials for ionization, etc. Electroncapture detectors, heretofore employing radioactive isotopes as a sourceof ionization, have been widely used in the analysis of electroncgativematerials such as pesticides, steroids and amino acids. These detectorsare particulariy advantageous because of their great sensitivity andhigh degree of selectivity. However, the radioactive isotopes presentboth health hazards and require government licenses, requiringspecialized handling. Also, the upper operating temperatures of suchdetectors have been a serious limitation, for example where moresensitive detectors have employed tritium sources. Tritium employed indetector structure vaporizes appreciably above 220 C, Accordingly, it isan object of this invention to provide an electron capture detectorwhich does not employ radioactive isotopes.

Another object of the invention is to provide such a detector having anupper operating temperature limited only by the materials ofconstruction, for example 400 to 500 C.

Still another object of the invention is to provide such a detector inwhich the source of ionization is a combination of electric dischargeand ultraviolet light.

A further object is to provide such a detector employing ceramic andmetal body portions to permit operation at higher temperatures.

A still further object of the invention is to provide such a detector inwhich the electron current may be varied over several orders ofmagnitude and which may operate at current levels greatly exceedingthose presently available in detectors employing radioactive sources.

In carrying out the invention in one form thereof a chamber is providedhaving a discharge portion with a pair of discharge electrodes and asensing portion. An apertured anode and cathode are positioned in thesensing portion and a restriction is provided connecting the twoportions of the chamber. A source of potential, to establish a gradientnot exceeding .7 of a volt per centimeter per millimeter of mercurypressure, is connected across the anode and cathode and a second sourceis connected to create a discharge across the discharge electrodes. Agas which is chemically inert with respect to the materials of thedetector and its other contents is injected into the discharge portionto flow past the dis-charge electrodes through the restriction and thenthrough the cathode. A carrier gas which may contain a sample gascapable of capturing electrons is injected through the anode into thesensing chamber. The two gases are exhausted from a point between thecathode and anode. The relative potential of the electrodes is set suchthat the discharge electrode field is negative and the anode positivewith respect to the cathode to enhance the flow of electrons from thedischarge electrodes into the sensing portion of the detector. Typicalgases which may be used in the discharge portion are hydrogen, nitrogen,carbon dioxide and the noble gases. For carrier gases examples arecarbon dioxide, nitrogen and a mixture of the noble gases and otherssuch as argon and methane.

The novel features which are believed to be characteristic of theinvention are set forth with particularity in the appended claims. Theinvention itself, however, together with further objects and advantagesthereof, can best be understood by reference to the followingdescription taken in connection with the accompanying drawingillustrating one embodiment of the invention in cross section.

Turning now to the drawing, a detector is illustrated having acylindrical sensing chamber 10 contained in a cylindrical housing 12which may be made from alumina, for example. Opposite ends of thesensing portion 10 are defined by a cathode 14 and an anode 16 which maybe made of stainless steel mesh (approximately 200 mesh). The chamber 10may be approximately A2 inch in diameter and ,3 inch from the anode tothe cathode and has an exhaust port 18 which may be approximately /sinch OD. and positioned about /3 of the way from the cathode to theanode. An inlet port 20 is provided for passing carreir gas, which maycontain a sample, through the anode 16 into the sensing chamber 10. Alead 22 is taken from the cathode 14 through the housing 12 to providean electrical connection to a source of potential 24 and thence toground. A second lead 26 is taken from anode 16 through housing 12 toprovide an electrical connection to an amplifier 23 and then to arecorder 30. Potential source 24 thus establishes the required gradientbetween the anode 16 and the cathode 14.

The housing 12 is attached to a housing 32, which may be of stainlesssteel, by means of a cylindrical collar 34, which may be made of nickel.Housing 32 has inserted within it an inner housing 36, which may be ofquartz or ceramic, which defines a discharge chamber 38. Dischargechamber 38 contains two discharge electrodes 40 which may have platinumtips of approximately .03 inch in diameter spaced approximately .03 inchapart. The discharge electrodes 48 are sealed in bushings 42 which maybe of alumina or cezamic and electrodes: 40 may be approximately inchfrom cathode 14. Bushings 42 each have a collar 44, which may be ofnickel, on top of which a gasket 46, which may be of silver, is placed.A seal is provided by screwing the retaining nuts 48, which may be ofsteel, down on the gaskets 46.

The inner housing 36 is held in place in the housing 32 by means of agasket 50 which may be of silver and a back-up plate 52 which may be ofstainless steel. A second back-up plate 5'4, which may also be ofstainless steel, forces back-up plate 52 against gasket 5 and is held inplace by means of screws 56 which pass through housing 32. A tube 58 isinserted in back-up plate 52 and provides an inlet port which extendsthrough the back-up plate 52 and washer 50 and through housing 36 intothe discharge chamber 38, through which a gas for the discharge chamber,such as hydrogen, nitrogen, carbon dioxide or a noble gas may beadmitted. The discharge chamber 38 may have dimensions of approximately.18 inch in drameter by .36 inch long.

A flow path is provided through a restriction 60 which may beapproximately .7 inch in diameter, from the discharge chamber 38 andthrough an intervening space 62 between the discharge chamber 38 and thesensing chamber 10 of the detector, such that the gas sweeping throughtube 58 may pass the discharge electrodes 40 in the chamber 38 and sweepelectrons through the restriction 60 into the sensing chamber 10 pastcathode 14.

Discharge electrodes 40 are connected together by leads 64 and 66, whichpass through the bushings 42, through a source of potential 68 and aresistor 70, which may be for example approximately -40,000 ohms, inorder to cause a discharge between the electrodes. The terminal 72between source of potential 68 and resistor 70 is grounded as shown.

The source of potential 68 may be approximately 200 to 400 volts, whenusing helium as a discharge chamber gas; and source 24 may range from 0to 500 volts for example with polarities as indicated. With the sourcesof potential of the magnitudes described, and the other typicaldimensions given, a gradient is established across the anode 16 andcathode 14 of less than approximately .7 of a volt per centimeter permillimeter of mercury pressure in the chamber which may operate aroundatmospheric pressure. This is done in order to operate in the electroncapture mode avoiding electron multiplication. The flow of gas past thedischarge electrodes 40 and through the restriction 60 serves to preventthe carrier gas, containing any sample, flowing through port 20 fromcoming into contact with the platinum-tipped discharge electrodes 40.This then prevents the discharge electrodes from becoming contaminated.The relative potentials of the field between the discharge electrodes40, cathode 14 and anode 16 serve to accelerate electrons generated inthe are at discharge electrodes 40 through cathode 14 to anode 16 wherethey are collected. Resistor 70 is employed specifically to accomplishthis by establishing the discharge electrode nearest ground potentialalso negative with respect to the cathode 14. An additional effecttaking place within the detector occurs in that photons generated at thedischarge electrodes 40 radiate through the restriction 60 and strikethe cathode 14 releasing additional electrons. The flow of gas from thedischarge chamber 38 through the cathode 14 also serves to preventcontamination and modification of the cathode 14 with respect to itsphotoelectric emission properties.

Helium is one preferred gas used in the discharge chamber 38 because ofthe relatively low potential required to strike an arc. A preferredcarrier gas for the sensing chamber is nitrogen which is made to flowcounter to the stream of electrons passing between cathode 14 and anode16. When electronegative materials such as pesticides, steroids andamino acids that exhibit the property of capturing some of the electronsout of the flow are contained in the nitrogen, the electron flow, thedensity of which may be conveniently modified by controlling the appliedpotentials, is diminished.

The decrease in the electron current flowing between cathode 14 andanode 16 yields an indication of the type, presence and amount ofelectronegative sample gas contained in the carrier when calibrated withtime after injection of the sample into a chromatograph column which isthus indicated by a reduction in the current passing through theamplifier 28 and resulting variation recorded on the recorder 30. Thesuggested materials of construction also permit operation of thedetector at temperatures up to 400 to 500 C., which is ab ve t e columnperatures normally employed. The detector then can operate in an oven toavoid condensation of sample.

While the principles of the invention have now been made clear in theillustrative embodiment, there will be immediately obvious to thoseskilled in the art many modifications in structure, arrangement,proportions, elements and components used in the practice of theinvention and otherwise, which are particularly adapted for specificenvironments and operating requirements, without departing from theseprinciples. The appended claims are therefore intended to cover andembrace any such modifications that fall within the limits only of thetrue spirit and scope of the invention.

What is claimed is:

1. An electron capture detector comprising, a chamber having a dischargeportion and a sensing portion, a pair of discharge electrodes positionedin said discharge por tion, means for connecting a source of potentialacross said discharge electrodes, an anode and a cathode positioned insaid sensing portion, means for connecting a source of potential acrosssaid anode and cathode to establish a gradient not exceedingapproximately seven tenths volt per centimeter per millimeter of mercurypressure therebetween, means for admitting a gas chemically inert withrespect to the detector and its other gas contents into said dischargeportion to flow past said discharge electrodes and said cathode, meansfor admitting a carrier gas for containing a sample into said sensingportion to flow past said anode, said carrier gas being inactive withrespect to said sample and having substantially no electron capturecapability, and means for exhausting said gases from a point betweensaid anode and cathode.

2. An electron capture detector for a gas chromatog raph comprising, achamber having a discharge portion and a sensing portion, a pair ofdischarge electrodes positioned in said discharge portion, means forconnecting a source of potential across said discharge electrodes, ananode and a cathode positioned in said sensing portion, means forconnecting a source of potential across said anode and cathode toestablish a gradient not exceeding approximately seven tenths volt percentimeter per millimeter of mercury pressure therebetween, means foradmitting a noble gas into said discharge portion to flow past saiddischarge electrodes and said cathode, means for admitting a carrier gasinto said sensing portion to flow past said anode, and means forexhausting said gases from a point between said anode and cathode.

3. An electron capture detector for a gas chromatograph comprising, achamber having a discharge portion and a sensing portion, a pair ofdischarge electrodes positioned in said discharge portion, means forconnecting a source of potential sufiicient to cause an arc across saiddischarge electrodes, an apertured anode and an apertured cathodepositioned in said sensing portion, means for connecting a source ofpotential across said anode and cathode to establish a gradient notexceeding approximately seven tenths volt per centimeter per millimeterof mercury pressure therebetween, means for admitting a gas chemicallyinert with respect to the detector and its other gas contents into saiddischarge portion to flow past said discharge electrodes and saidcathode, means for admitting a carrier gas for containing a sample gashaving an afiinity for electrons into said sensing portion to flow pastsaid anode, said carrier gas being inactive with respect to said sampleand having substantially no electron capture capability, means forexhausting said gases from a point between said anode and cathode, andsaid sources of potential establishing the field between said dischargeelectrodes negative and said anode positive with respect to saidcathode.

4. An electron capture detector for a gas chromatograph comprising, achamber having a discharge portion and a sensing portion, a pair ofdischarge electrodes positioned in said discharge portion, means forconnecting a source of potential sutficient to cause an arc across saiddischarge electrodes, an apertured anode and an apertnred cathodepositioned in said sensing portion, means for connecting a source ofpotential across said anode and cathode to establish a gradient notexceeding approximately seven tenths volt per centimeter per millimeterof mercury pressure therebetween, means for admitting a noble gas intosaid discharge portion to flow past said discharge electrode and throughsaid cathode, means for admitting a carrier gas for containing a samplegas having an affinity for electrons into said sensing portion to flowpast said anode, means for exhausting said gases from a point betweensaid anode and cathode, and said sources of potential establishing thefield between said discharge electrodes negative and said anode positivewith respect to said cathode.

5. An electron capture detector for a gas chromatograph comprising, achamber having a discharge portion and a sensing portion, a pair ofdischarge electrodes positioned in said discharge portion, means forconnecting a source of potential sutficient to cause an arc across saiddischarge electrodes, an apertured anode and an apertured cathodepositioned in said sensing portion, means for connecting a source ofpotential across said anode and cathode to establish a gradient notexceeding approximately seven tenths volt per centimeter per millimeterof mercury pressure therebetween, means for admitting helium gas intosaid discharge portion to flow past said discharge electrodes andthrough said cathode, means for admitting nitrogen gas for containing asample gas having an afiinity for electrons into said sensing portion toflow past said anode, means for exhausting said gases from a pointbetween said anode and cathode, and said sources of potentialestablishing the field between said discharge electrodes negative andsaid anode positive with respect to said cathode.

6. An electron capture detector for a gas chromatograph comprising, achamber having a discharge portion and a sensing portion, a pair ofdischarge electrodes positioned in said discharge portion, means forconnecting a source of potential sufiicient to cause an arc across saiddischarge electrodes, an apertured anode and an apertured cathodepositioned in said sensing portion, means for connecting a source ofpotential across said anode and cathode to establish a gradient notexceeding approxmiately seven tenths volt per centimeter per millimeterof mercury pressure therebetween, means for admitting helium gas intosaid discharge portion to flow past said discharge electrodes andthrough said cathode, means for admitting nitrogen gas for containing asample gas having an aflinity for electrons into said sensing portion toflow past said anode, means for exhausting said gases from a pointbetween said anode and cathode, said sources of potential establishingthe field between said discharge electrodes negative and said anodepositive with respect to said cathode, and the centers of said anode andcathode and the center point: between said discharge electrodes beingsubstantially in a straight line.

7. An electron capture detector for a gas chromatograph comprising, adischarge chamber and a sensing chamber, a pair of platinum tippeddischarge electrodes positioned in said discharge chamber, means forconnecting a source of potential sufficient to cause an arc across saiddischarge electrodes, an anode of stainless steel mesh positioned insaid sensing chamber, a cathode of stainless steel mesh positionedacross one end of said sensing chamber, a necked down chamber connectingsaid discharge and sensing chambers, means for connecting a source ofpotential across said anode and cathode to establish a gradient notexceeding approximately seven tenths volt per centimeter per millimeterof mercury pressure therebetween, means for admitting a helium gas intosaid discharge portion to flow past said discharge electrodes and saidcathode, means for admitting a mixture of nitrogen and a sample gashaving an afiinity for electrons into said sensing portion to first flowpast said anode, means for exhausting said gases from a point betweensaid anode and cathode such that said mixture remains substantially outof contact with said cathode to avoid contamination of said cathode,said sources of potential establishing the field between said dischargeelectrodes negative and said anode positive with respect to saidcathode, and the centers of said anode and cathode and the center pointbetween said discharge electrodes being sub stantially in a straightline.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, PrimaryExaminer. STANLEY D. SCI-ILOSSER, Examiner. R. JUDD, Assistant Examiner.

